TRANSISTORIZED PYROlMETER WITH AND WITHOUT FEEDBACK

The measurement of missile and rocket sur­face temperatures in their operational en­

vironments is a frequent requirement in the work of the APL hypersonic research facility. The haz­ardous nature of this research, together with the short operating times involved, requires that these measurements be automatic and that they be con­trolled by an operator remote from the test area. To meet these requirements, a completely transis­torized recording pyrometer for measuring bright­ness temperatures was developed by S. A. Elder. * The pyrometer measures surface temperature by sensing the amount of radiation emitted from the surface at a fixed wavelength (1.4 microns) . Calibration is performed by means of a "black body" source, an ideal radiator, for which the radiant power spectrum and temperature are re­lated by Planck's law.

The recording pyrometer system consists of the pyrometer, a control box, and connecting cables. The principal components of the pyrometer are a telescope lens, infrared filter, chopper wheel, pinhole, condenser lens, and germanium photo­transistor. A narrow pencil of light carries an image of the hot source from the port in the nozzle wall (as small as X 6 in.) and feeds it through this system onto the photo transistor. The condenser lens is required to smooth out the photosensitive irregularities of the photo transistor to provide a uniform light-sampling spot. This is the same prin­ciple used in photo enlargers to avoid imaging the light filament onto photosensitive paper.

The temperature range of the pyrometer is affected by the dynamic voltage range of the elec­tronics and by the efficiency of the optical system. With the telescope objective wide open, the lowest temperature measurable by the instrument is ~725 °K, with a range extending up to some maximum set by the linear limit of the voltage amplifier. In principle, temperatures in excess of 3000 OK could be measured on the pyrometer by a suitable choice of attenuators, though in practice it has been used only below 2500 0 K (~4000°F).

• s. A. Elder , " D esigning Phototransistor P yrometers wit h and without Feedback, " Electronics, 34, 1961, 56-60; and " A Completely Transis­torized R ecording P yrometer ," Tem perature-Its Measurement and Control in Science and Industry, 3, Part 2, 1962 ,873- 877.

May - June 1963

This pyrometer system has certain advantages over systems based on phototube detectors; chief among them are its almost complete insensitivity to shock and vibration, and its compactness and low cost. Also, the fact that it operates at infrared wavelengths gives it a wider temperature range than is possible with most phototubes. Under or­dinary laboratory conditions, the accuracy of the system is better than ± lOoC. Response times as short as 20 IJ.sec can be achieved with the photo­transistor currently in use.

In an improved version of the pyrometer, the electro-optical response of the photo transistor is stabilized by " photofeedback." Here, the photo­transistor becomes the first stage of a direct­coupled common-emitter cascade amplifier. The

CABLE TO CONTROL BOX

PREAM PLI FIER

PHOTOTRANSISTOR CONDENSER LENS

MOTOR

CHOPPER WHEEL~-._.,. (l ight modulator) FILTER

FllZlJ~- (passband I . IIL to I .7IL)

PO INT AT W HICH M EASUREMENT Is __ 1

MADE Q G AS FLOW

SchelUatic of a pyrolUeter screwed into the side of a rocket nozzle for throat-telUperature lUeasure­lUent.

17

feedback is introduced in the form of an infrared light signal. A subminiature tungsten bulb acting as an electro-optical transducer re-introduces into the window of the photo transistor an optical signal proportional to the electrical signal at the output of the amplifier. Base bias for the photo transistor is supplied by the DC component of the light from the lamp, which is run at a steady 12-ma current.

The photofeed back version of the pyrometer gives greater precision as well as greater calibra­tion stability . The voltage calibration is reproduc­ible to better than 0.1 %, giving a temperature that is reproducible to better than ± 1 °C at 1000° K and ±4°C at 2000 0K. Even greater precision could be achieved by redesigning for a larger amount of feedback, but stability of the feedback lamp characteristics may impose a practical limit. The threshold temperature of this version is ~8400K.

The most important advantage of the feedback version of the pyrometer, as previously noted, is that after an initial absolute calibration, it needs no further optical calibration and is therefore ready for field use under any conditions. Another advantage is that feedback improves the linearity of the photo transistor response.

An accompanying figure shows how the pyrome­ter is being used currently to measure throat-

Recording pyrometer designed for use by the Flight Research Group.

surface temperature in a rocket motor. By mount­ing the pyrometer directly to the rocket-nozzle wall (a capability made possible by its small size and weight), optical alignment errors due to thermal expansion can be avoided. An optical pyrometer is preferable to a thermocouple in this application since it has no thermal lag or lead conduction loss.

Other groups in the Laboratory are finding uses for the transistorized pyrometer. A conven­iently sized unit using the same basic optical and electronic design, which has been adapted by J. M. Akridge for use by the Flight Research Group, is illustrated.

SURVIVAL etropC)litan-~rea clear Attacks

On May 25, 1961, in his message to Congress on "Urgent National Needs," the President called for a new and revitalized · civil defense program to protect the civilian population of the United States from the hazards of nuclear war. Since that time, many pamphlets, books, and articles have appeared urging the population to begih providing the means for shelter-

ing itself from the residual radioac­tivity ("fallout") that would result from a nuclear attack.

when quite remote from the actual ground zero of a large-yield thermo­nuclear detonation. They can pro­vide all the immediate protection that is required for a large segment of the population (20 million rural inhabi-

18

It is agreed * that "fallout shelters" will provide useful protection only

• The author acknowledges tha t the factua l d a ta upon which this study is based were derived from U . S. Government a nd other publica tions lis ted at the conclusion of this paper. The sta tements herein do not con-

stitu te a specific set of recommend a tions. but are a n aggregation of facts from which the reader m ay deter mine his own course of action .

APL T echnical Digest